Existing devices and measurement procedures of air flow speed use multiple technologies. Among the most widespread are Pitot Tube-type anemometers with pressure sensors, those with propellers, thermal anemometers, and more recently ultrasound sensors.
Pitot Tube technology relies on a measure of differential pressure between the total pressure and the static pressure of a flow. This differential pressure is proportional to the dynamic pressure (ΔP=α*½*ρ*V2), where V is the speed of gaseous flow. Pitot Tubes are particularly adapted to high speeds even if the quality of the measurement is very sensitive to the direction of the flow.
For devices with propellers or turbines located in an airflow, the speed of the flow is determined by the rotational speed of the rotary element. These devices can provide a very precise measurement for average speeds and when the flow is directed along their rotational axis. Mechanical wear due to the propeller's rotational movements as well as the loss of charge generated are the principal disadvantages of these devices.
Another known device is the hot wire device for which the measurement process takes place by the measurement of a current intended to maintain the temperature of a wire cooled by an air flow and the comparison of the value of this current with a nominal temperature value obtained without air flow.
Thermal anemometers use the temperature of a sensing element that can be regulated by current, by temperature or by constant temperature differential. The regulation is the image of the flow speed. They allow measurement of very weak flows such as drafts and are, for this reason, well-adapted to use in air conditioning and ventilation.
An example of a constant temperature anemometer is provided by the document U.S. Pat. No. 4,503,706.
The hot wire device can be omni-directional, but in this case it gives no indication of the direction of the airflow, or it can be streamlined to favor one measurement direction and be capable of measuring, in this case, a flow along to a reduced angular sector.
Even if certain ones allow for the complete determination of a flow's directional characteristics, the different technologies presented above do not allow for the exploration of more than a small angular sector.